This invention relates to an abnormal combustion-detecting device and a combustion control device for internal combustion engines, and more particularly to an abnormal combustion-detecting device which is adapted to detect abnormal combustion caused by so-called hot surface ignition, and a combustion control device which is adapted to supress abnormal combustion detected by the former device.
In an internal combustion engine equipped with an ignition timing control system, the ignition timing, at which is ignited an air-fuel mixture supplied into an engine cylinder, is set to a value suited for operating conditions in which the engine is operating, so as to optimize the driveability of the engine as well as emission characteristics thereof. However, it is known that, during actual operation of the engine, there can occur so-called preignition, which is a kind of abnormal combustion that the mixture starts to burn in advance of the set ignition timing. The preignition possibly leads to poor driveability of the engine or even serious damage to same. Furthermore, in recent years, there is a tendency toward increased possibility of occurrence of preignition with recent improvement in engine performance and increase in engine output torque, as well as recent wider use of methanol fuels in engines.
Abnormal combustion is primarily attributable to so-called hot surface ignition that a mixture compressed within the engine cylinder becomes ignited by heat of the hot surface of the overheated spark plug or cylinder inner wall in advance of the usual spark ignition by the spark plug. Particularly, use of a methanol fuel (alcohol mixture fuel), which has recently been developed for the purposes of environmental protection as well as energy saving, in an internal combustion engine further increases the possibility of hot surface ignition or abnormal combustion.
In an attempt to avoid such abnormal combustion, an abnormal combustion-detecting device has been proposed, e.g., by Japanese Provisional Patent Publication (Kokai) No. 55-75629, which is adapted to detect ionic current caused by burning of a mixture within the engine cylinder to determine a combustion state therewithin.
Hot surface ignition is classified into preignition, i.e., ignition which takes place at a timing before normal spark ignition timing, and postignition, i.e., ignition which takes place at a timing behind normal spark ignition timing when spark ignition is not effected at normal timing. It is known that the timing of occurrence of flame caused by postignition progressively advances, as the temperature of the cylinder inner wall etc. rises, and eventually into preignition. Based on this fact, it is possible to predict the possibility of occurrence of preignition by detecting postignition occurring during operation of the engine. However, in actuality, the occurrence of postignition cannot be recognized by detecting merely a timing at which the mixture starts to burn within the engine cylinder, because flame due to the postignition appears only after the mixure has partly started to burn due to the spark ignition by the spark plug.
On the other hand, the preignition is a phenomenon that a mixture starts to burn within the engine cylinder in advance of spark ignition by the spark plug. That is, when preignition occurs, flame caused thereby is propagated throughout the interior of the engine cylinder to cause explosion therein when the engine is still on the compression stroke, thereby possibly doing serious damage to engine parts in the worst case. Unlike postignition, the occurrence of preignition can actually be recognized by detecting the timing at which the mixture starts to burn, because the flame caused by preignition appears before the mixure starts to burn upon spark ignition by the spark plug. However, it is requisite to prevent occurrence of preignition, because once preignition occurs, the engine may be seriously damaged.